The style loss of Gatys et al. [1]} had previously been proposed by the same authors for the purpose of texture synthesis [2]}, and that work was inspired by the parametric matching of hand-crafted features (for texture synthesis) proposed by Portilla and Simoncelli [3]}. The loss is based on Gram matrices computed over the spatial dimensions of \(\Phi (X)^l\) , the tensor of neural activations in layer \(l\) . If \(\Phi (X)^l\)  has \(F\)  feature channels then the gram matrix at that layer \(\mathcal {G}^l\)  is a positive semi-definite matrix \(\mathcal {G}^l\in \mathbb {R}^{F\times F}\) . Each entry of such a matrix is defined as:
\(\mathcal {G}^l_{i,j} = \sum _k \Phi (X)^l_{i,k} \cdot \Phi (X)^l_{j,k}\) 
